Skin rejuvenation is a medical aesthetic treatment, in which energy is applied to selected areas of the skin surface and/or to subcutaneous layers of the skin in order to achieve an improvement in the appearance of the treated skin. The most popular form of skin rejuvenation is the application of an amount of energy to the skin to heat target tissue to temperatures sufficiently above normal body temperature to induce desired effects in the tissue. The effects may be tissue damage, coagulation, ablation, destruction and necrosis. The specific effects achieved depend on the tissue, the temperature, and the period of time the tissue is maintained at the high temperature. This treatment improves the appearance of the skin by tightening the skin and reducing wrinkles, and by promoting regeneration in the skin layers and subcutaneous tissue.
Non-invasive delivery of energy to internal tissues has been done by directing electromagnetic energy or ultrasound energy to the skin surface. Electromagnetic radiation from a broad range of wavelengths has been used for heating the skin, including optical radiation, frequencies above 30 GHz, frequencies between 300 MHz to 30 GHz, and radio frequency (RF) energy. Typical RF frequencies used for skin treatment are between 100 kHz and 10 MHz. The technology, propagation through the body, interactions with the skin, and the effects on tissues are different for each part of the spectrum. Simultaneous application of optical energy and RF energy has also been used to treat skin.
U.S. Pat. No. 5,405,368 discloses the use of flash lamps for skin treatment. U.S. Pat. No. 5,964,749 describes a method and apparatus for treating skin which includes applying pulsed light to the skin to heat the skin in order to effect shrinking of collagen within the skin, thereby restoring the elasticity of the collagen and of the skin. The epidermis and outer layers of the skin may be protected by cooling with a transparent substance, such as ice or gel, applied to the skin surface. The temperature distribution within the skin is controlled by controlling the delay between the time the coolant is applied, and the time the light is applied, by controlling the pulse duration, applying multiple pulses, filtering the light and controlling the radiation spectrum. Preferably, the spectrum includes light having a wavelength in the range of 600-1200 nm. The pulsed light may be incoherent, such as that produced by a flashlamp, or coherent, such as that produced by a laser, and may be directed to the skin using a flexible or rigid light guide. U.S. Pat. Nos. 6,662,054 and 6,889,090 disclose the application of RF energy for subcutaneous treatment. U.S. Pat. No. 6,702,808 discloses a combination of light and RF energy for skin treatment. U.S. Pat. No. 5,871,524, describes application of radiant energy through the skin to an underlying subcutaneous layer or deeper soft tissue layers.
The main limitation on non-invasive skin treatment is the ability to transfer the energy through the outer layers of the skin and concentrating it to the required level in the target tissue, with minimal collateral damage to the surrounding tissue, including the tissue through which the energy must pass on its way to the target tissue. The solutions are based either on selective cooling or focusing of radiation. Focusing is possible when the wavelengths are sufficiently short, for example with optical radiation, millimeter and sub-millimeter waves, and high frequency ultrasound. Optical radiation is scattered inside the skin, so it is difficult to focus efficiently. Laser light is preferred in order to enable better focusing. U.S. Pat. No. 5,786,924 discloses a laser system for skin treatment. Published U.S. patent application Ser. No. 10/888356 to De Benedictis et al., having the publication number 2005/0049582, discloses using one or more light sources to generate microscopic treatment zones in skin in a predetermined pattern. The advantage of this approach is that the damaged tissue is localized to small volumes surrounded by healthy tissue, so that skin regeneration is faster.
High intensity focused ultrasound (HIFU) technology for non-invasive skin treatment is disclosed, for example, in U.S. Pat. Nos. 6,325,769 and 6,595,934. The last patent discloses the application of an array of focused ultrasound transducers, which generates an array of lesions in the skin or subcutaneous layers, with advantages similar to those disclosed in the above mentioned US Patent Application Publication 2005/0049582 but with minimal damage to the outer skin layer due to the focusing of the radiation. The resolution of the focusing of electromagnetic energy is limited by diffraction laws to about half of the wavelength. For less than a 0.5 mm focal dimension, a wavelength shorter than 1 mm is required. Although the application of electromagnetic energy at sub-millimeter wavelengths may have several advantages, generating sub-millimeter radiation is impractical for skin treatment due to its high cost. In RF applications, voltages and currents can be induced in body tissues by applying electrodes to the skin surface, which do not propagate as waves but rather fall into the quasi-static regime of the Maxwell equations. RF applications for non-invasive skin treatment are disclosed, for example, in U.S. Pat. Nos. 6,662,054, 6,889,090, 5,871,524. Typical RF frequencies used are between 100 kHz and 10 MHz. At these frequencies, the wavelength, which is between 3000 m and 30 m is much larger than any relevant dimension of the treated tissue. An AC current is induced in the skin by the applied AC voltage, generally obeying Ohm's law. RF technology is relatively simple and inexpensive, and very effective in transferring energy to a tissue. However it is difficult to localize it to a specific tissue layer. One method to generate selectivity is by cooling the skin surface, thereby creating a temperature gradient from the outside to the internal layers. Such a method is disclosed in U.S. Pat. No. 5,871,524.